Vitamin B12 deficiency in adults: A contemporary guide to recognition, diagnosis, and evidence-based management

Vitamin B12 (cobalamin) deficiency is a prevalent, clinically heterogeneous disorder with significant potential for hematologic, neurologic, and psychiatric morbidity. Its insidious onset and nonspecific presentation frequently lead to diagnostic delays, particularly as neurological sequelae can occur in the absence of anaemia. This review synthesises current evidence and expert consensus to provide a structured, evidence-based framework for clinicians. We delineate the complex pathophysiology of B12 deficiency, spanning from dietary insufficiency and autoimmune pernicious anaemia to drug-induced malabsorption. A systematic approach to differential diagnosis is presented, contrasting B12 deficiency with key mimickers. Diagnostic strategies are evaluated, highlighting the superior specificity of metabolic biomarkers like methylmalonic acid (MMA). Finally, we review definitive treatment pathways, critically analysing the robust evidence supporting high-dose oral supplementation as a first-line therapy for most patients, including those with malabsorptive conditions. This guide aims to optimise timely diagnosis, prevent irreversible complications, and reflect modern, patient-centred treatment paradigms.